Electromagnetic waves having a wavelength shorter than ultraviolet rays, say, between 10 and 0.01 nm are generally called X-rays. Among them, those having a wavelength longer than about 0.3 nm are called soft X-rays while those having a wavelength shorter than 0.3 nm are called hard X-rays. Those having shorter wavelengths than X-rays are called .gamma.-rays.
X-rays are ordinarily produced by bombarding accelerated electron beams against a metal. In order to produce X-rays by this principle, sealed X-ray tubes or rotary anode type X-ray tubes are used. To produce soft X-rays, an electron beam-excited soft X-ray source or a plasma soft X-ray source is used.
Such conventional X-ray sources were extremely low in X-ray generating efficiency. In fact, they can produce X-rays having a power of only several percent or less of the power exerted on the target with most of the power being converted into heat. Thus, a large amount energy is wasted. If laser beams are utilized to produce X-rays as with plasma soft X-ray sources, laser beams are wasted as heat. Thus, in order to increase the amount of produced X-ray, it is necessary to use powerful laser beams. Also, these X-ray sources tend to produce X-rays having a wide spectrum other than characteristic X-rays. This worsens the X-ray generating efficiency if it is necessary to produce X-rays with monochromatic spectrum of energy.
Now turning the subject to the field of astronomy dealing with gravitational waves, Einstein predicted in his famous general theory of relativity the existence of gravitational waves which are considered to be produced by explosion of supernovas. But as of today, such gravitational waves have not been observed directly, though trials have been made to develop antennas which can pick up such gravitational waves.
Gravitational wave antennas are designed to pick up "spatial distortion due to gravitational waves". The spatial distortion is extraordinarily low in level. In fact, the distortion level is so low that it is impossible to detect such distortion with ordinary industrial equipment. Such a distortion will be observed e.g. with a laser interferometer which will be formed by applying laser beams to a Michelson interferometer.
Such a laser interferometer uses an ultra-high-performance performance optical resonator provided in the optical path of laser beams so that it can detect ultra-small deflections. The optical resonator is a Fabry-Perot resonator having two oppositely arranged mirrors. The mirrors used are ones having a surprisingly high reflectivity. The higher the reflectivity of the mirrors used, the larger the amount of energy which the optical resonator can accumulate. When synchronized with light having a predetermined frequency, the resonator can accumulate a high-intensity light beam.
On the other hand, an X-ray generator utilizing the Compton backward scattering effect is disclosed in U.S. Pat. No. 4598415. It has a laser oscillator comprising two reflecting mirrors and a laser disposed between the mirrors. Electrons accelerated by an accelerator are put in orbit and let collide with the laser beams that reciprocate between the reflecting mirrors. When they collide, X-rays having a narrow frequency band are produced due to the Compton scattering.
The X-rays and .gamma.-rays emitted from this X-ray generator due to the Compton scattering are narrow in spectrum and monochromatic. Also the direction in which they are emitted is concentrated within the solid angle of forward 1/.gamma. (.gamma. is the Lorentz factor of the electron beams).
On the other hand, with this method, the sectional area of collision between electrons and photons is extremely small, so that it is impossible to sufficiently increase the radiation dose. In order to solve this problem, it was proposed to generate X-rays in a resonator as in the above-identified U.S. Patent.
But since the output of the X-ray generator of the type disclosed in the U.S. Patent is determined by the saturation level of laser gain in the resonator, it is impossible to increase the output above a predetermined value no matter how high the reflectivity of the reflecting mirrors. Also, these media necessarily have a damage threshold of the laser media according to the laser intensity. Such a threshold also limits the internal power. For these reasons, the reflectivity of the resonator is at most 99-98%. Moreover, since only a resonator is used as a laser, its output is not so large. It is impossible to use a large-output laser utilizing an oscillating/amplifying system.
In spite of the fact that an optical resonator having a surprisingly high reflectivity of 99% or higher is available, conventional X-ray generators do not require an optical resonator having such a high reflectivity. Thus, even if such a high-performance resonator is used with a conventional X-ray generator to produce X-rays utilizing laser beams, it is impossible to produce high-intensity X-rays or .gamma.-rays with high efficiency.
The inventors have found out that powerful X-rays or .gamma.-rays can be obtained by accumulating laser beams in a photon accumulating cavity by means of ultra-high-reflectivity mirrors and bombarding electron beams against the laser beams.
But in order to produce high-power X-rays or .gamma.-rays, it is necessary not only to improve the efficiency of interaction between the electron beams and the laser beams, but to properly set various other conditions such as the conditions of electron beams and laser beams introduced into the interaction area and the energy intensity of the laser beams.